(1) Field of the Invention
The present invention relates to storage batteries, such as Ni—Cd batteries, Ni-MH batteries, and Li-ion batteries. In particular, the present invention relates to a battery having a collector plate welded to one end of electrode plates included in a spirally wound electrode assembly.
(2) Description of the Related Art
In recent years, alkaline storage batteries, such as Ni—Cd batteries and Ni-MH batteries, and Li-ion batteries and the like are used as batteries for electric vehicles, electric motorcycles, power-assisted bicycles and so on, which require large current. An alkaline storage battery used for this purpose is required to have high output characteristics and a high energy density. To achieve the high output characteristics, current-collector parts are required to be of low resistance, and the electrode substrate at the electrode plate ends and the collector plate are required to be in close contact, for instance. Also, the welding strength between the collector plate and the electrode substrate has to be high to prevent the collector plate from breaking away from the electrode substrate due to vibration during use.
In a usual manufacturing process of the alkaline storage battery, firstly, a positive electrode plate and a negative electrode plate, which sandwich a separator, are spirally wound together to form an electrode assembly. An electrode substrate of the negative electrode plate included in the electrode assembly is welded to the negative collector plate, and the electrode substrate of the positive electrode plate included in the electrode assembly is welded to the positive collector plate. Next, the electrode assembly is housed in a metal casing which also serves as a negative terminal, and the negative collector plate is welded to the bottom of the casing. Meanwhile, a collector lead extended from the positive collector plate is welded to the bottom of a closure construction which also serves as a positive terminal. Then, the casing is filled with an electrolyte, and the closure construction is attached to an opening part of the casing so that an insulating gasket is sandwiched therebetween. In this way, the manufacturing process of an enclosed alkaline storage battery is completed.
To further improve the energy density of the alkaline storage battery, nickel foam is used for the Ni positive electrode substrate by which the amount of the enclosed active material is increased. Since the nickel foam is highly porous and of a low density, it is difficult to directly weld the nickel foam to the positive electrode assembly. However, Document 1 (Japanese Laid-open. Patent Application Publication No. 11-149914) suggests welding a ribbon-shaped tab at the end of the nickel foam, and welding the tab to the positive electrode assembly.
When this method is adopted, the tab is additionally required, which means that the number of required parts is increased. Also, the welding process is required. Accordingly, the manufacturing cost increases. Regarding this problem, if a collector plate including a convex part having a V-shaped cross-section is used as disclosed by Document 2 (Japanese Laid-open Patent Application Publication No. 60-72160), and the welding method as disclosed by Document 3 (Japanese Laid-open Patent Application Publication No. 56-67166) is used, it is possible to directly weld the electrode assembly to the electrode substrate made from the nickel foam. The following more specifically describes this method.
As described in Document 2, a convex part 54 having a V-shaped cross-section is formed on one of main surfaces of the main body 51 (see FIG. 9) included in the collector assembly 50. Then, as FIG. 9A shows, a pair of electrodes R1 and R2 for welding (hereinafter called the welding electrodes) is disposed so as to sandwich, as viewed from above the main surface of the collector plate 50, a region where the convex part 54 and the end of the positive electrode plate (61c in FIG. 1) included in the electrode assembly 50 contact with each other. With this structure, as described in Document 3, the pair of the welding electrodes R1 and R2 is fed, and Joule heat is generated at the top 54a (the top extending linearly: herein after called “the linear top part”) of the convex part 54 and its vicinity due to the electrical resistance characteristics of the collector plate 50. Accordingly, the linear top part 54a of the convex part 54 and the end part of the positive electrode plate which contacts with the linear top part 54a are partially fused, and welded to each other.
In the welding method described above, the region where the Joule heat is generated is not limited to the linear top part 54a and its vicinity. This is because the amount of generated Joule heat is proportional to the current density of a current path relating to the welding, and higher Joule heat is generated where the current density is higher. For instance, on the path of the welding current, the current density in a region where the width is narrowest (e.g. a part X indicated by a dotted line in FIG. 9A) is higher than the current density in the other regions. Therefore, according to the collector plate 50, the heating value is highest at the part x, not at the linear top part 54a. In the worst case, the collector plate might be burnt off at the part X.
Furthermore, as the part Y (the edge of a groove having a substantially v-shaped cross-section) represented by a dotted line in FIG. 9A shows, if the length of the linear top part 54a and the current path relating to the welding is almost the same, the heating value is almost the same at any points on the part Y. Therefore, the collector plate might be burnt off at the part r in the same manner as in the case of the X part.
If such a burn-off happens, the collecting efficiency of the battery might be deteriorated, and the operating voltage at the discharge might be decreased. Further, another problem might because. Namely, when the burn-off happens, sparks might fly into the electrode assembly, and cause a short circuit.
As a measure for preventing such a problem, it is possible to reduce the welding current and prevent the burn-off. However, this makes the welding between the collector plate and the electrode plate insufficient, and highly possibly causes a problem of welding strength and current conduction.
As described above, there still is a problem to be solved regarding a method of welding between a collector plate and an end of an electrode assembly.